It seems there are a number of users having issues with 4.2.0. The downgrade to 4.1.2 is recommended at this time, you will not lose your collection.
Also, you are welcomed to join as beta testers of the fixed version from the link below.
Additionally, you can revert to a previous version of your database through an automatically created backup in the rekordbox folder:
Win 7/8/10: C:\Users\yourusername\AppData\Roaming\Pioneer\rekordbox
Rename datafile.edb to datafile.edb.old
Rename datafile.backup.edb to datafile.edb (removing .backup)
rekordbox automatically creates a backup each time you launch the software; if you have opened / closed v4.2.0 more than 3 times you may not be able to recover your database in this manner. Please check the date / time of the backup files in order to choose the most appropriate version.
As for the issue 2) Filter is not as sensitive as in rekordbox 4.1.2.
In 4.2.0, filter sensitivity was changed in accordance with how it is sensitive when using DJM-900NXS2.